This invention deals with primer and coating compositions. In particular, this invention deals with primer compositions which are used with room temperature curable silicone rubber and room temperature-curable silicone-modified organic rubbers in order to obtain excellent adhesion to a variety of substrates. In particular, it also deals with compositions, similar to the primer compositions, which are useful as coatings on various substrates. The compositions have the ability to be stored over a long period of time in the absence of moisture and they can be cured at room temperature in the presence of moisture. The compositions exhibit excellent adhesiveness with a variety of substrates.
In recent years, due to the superior durability of room temperature-curable silicone rubbers in comparison with other rubbers such as organic rubbers, they have become widely used as sealants in construction. Room temperature curable silicone-modified organic rubbers have recently been introduced and are also being used as sealants in construction. Such construction may employ various substrate materials, for example, metals such as aluminum, steel and stainless steel; aluminum when coated with acrylic resin, urethane resin or epoxy resin; hard inorganic materials such as glass, tile, stone and porous inorganic base materials such as mortar and concrete. Thus, a firm adhesion by the room temperature-curable silicone rubbers and room temperature-curable silicone-modified organic rubbers used as sealants has become an important problem.
A widely used method is the application of various primers to the substrate followed by the application of the room temperature curable silicone rubber or room temperature curable silicone-modified organic rubber. However, several of the above-mentioned substrates are difficult to adhere to, such as, for example, pure aluminum, surface-treated aluminum, stainless steel, aluminum coated with various resins and mortar. As a sealant, the silicone rubber or silicone-modified organic rubber peels off at the interface of the substrate before it deteriorates or loses its elasticity. Thus, primers which would maintain superior adhesive strength for lengthy periods is desired.
Conventionally, primers composed of epoxy resins and organofunctional silanes are well known. However, since the mutual miscibility of epoxy resins and the silane is poor, a durable and uniform adhesive film cannot be obtained.
The inventors discovered the present invention in an attempt to overcome the above-mentioned drawbacks. As a result, it was found that, by using an organotitanium acid ester to cure a silicone-modified epoxy resin which is itself obtained by a reaction between hydroxyl groups on an epoxy resin and the alkoxy groups of a silicone compound, a sturdy and transparent film can be formed. This film has superior adhesive strength when used as a primer for room temperature-curable silicone rubber and room temperature-curable silicone-modified organic rubber.
Thus, one aspect of this invention concerns primer compositions comprising (A) 100 parts by weight of a silicone modified epoxy resin which contains both epoxy groups and silicon-bonded alkoxy groups wherein the modified epoxy resin is obtained by contacting and reacting (a) a compound having the unit formula ##EQU1## wherein R is a substituted or unsubstituted monovalent hydrocarbon radical, X is an alkoxy radical having the formula R'O-- wherein R' is an alkyl radical of 1 to 4 carbon atoms or the radical R.sup.2 OR.sup.3 -- wherein R.sup.2 is an alkyl radical of 1 to 4 carbon atoms and R.sup.3 is a divalent alkylene radical of 1 to 3 carbon atoms; a has a value of 0 to 2; b has a value of 1 to 4 and the sum of a+b has a value of 1 to 4 with (b) an epoxy resin containing at least one epoxy group and at least one hydroxy group per molecule; and (B) 0.1 to 100 parts by weight of an organotitanium acid ester.
Component (A) is a primary component of the primer composition along with component (B).
Taking component (A) first, it should be noted that component (A) is prepared from two subcomponents, components (a) and (b). Component (a) prior to reaction with component (b) consists of a compound having the unit formula ##EQU2## R in component (a) is a substituted or unsubstituted monovalent hydrocarbon radical. The groups which have been found useful in this invention are, for example, alkyl groups such as methyl, ethyl, propyl and octadecyl; alkenyl groups such as vinyl and allyl and aryl groups such as phenyl. The substituted monovalent hydrocarbon radicals useful in this invention are those wherein the above disclosed unsubstituted monovalent hydrocarbon radicals are substituted by such groups as halogen, cyano, mercapto and hydroxyl groups or by organofunctional groups such as methacryloxy, acryloxy or the 3, 4-epoxycyclohexyl groups.
X in the above formula is an alkoxy radical having the formula R'O-- wherein R' is an alkyl radical of 1 to 4 carbon atoms or the R.sup.2 OR.sup.3 -- radical. Thus, X can be R'O-- such as methoxy, ethoxy or propoxy. X can also be R.sup.2 OR.sup.3 O-- wherein R.sup.2 is an alkyl radical of 1 to 4 carbon atoms and R.sup.3 is a divalent alkylene radical of 1 to 3 carbon atoms. R.sup.2 can be for example, methyl, ethyl or propyl and R.sup.3, for example can be methylene, ethylene or propylene. An example of R.sup.2 OR.sup.3 O-- can be, for example, methoxyethoxy--.
For purposes of this invention, a has a value of 0 to 2; b has a value of 1 to 4. Thus, included within the scope of this invention are compounds wherein R is not necessarily present. The reason for the values of a and b as set forth above is that there cannot be too few alkoxy groups in the resulting reacted product from (a) and (b). Components with too few alkoxy groups in the reacted product result in insufficient curing and insufficient adhesion in the final product. Thus, it is preferable that there be at least two X groups in (a) and at least 3 X groups be present in (A). Component (a) can be a silane, a polysiloxane or a siloxane oligomer. Polysiloxanes having a moderate degree of polymerization are preferred for this invention. The polysiloxane may be linear, branched chain or a network siloxane. In addition to the alkoxy radicals, this material can also contain small amounts of hydroxyl groups, halogen groups or hydrogen atoms.
Examples of silanes useful as component (a) are such silanes as methyltrimethoxysilane, dimethyldiethoxysilane, ethyltriethoxysilane, phenyltrimethoxysilane, vinyltrimethoxysilane, vinyltri(methoxyethoxy)silane, allyltripropoxysilane, gamma-methacryloxypropyltrimethoxysilane, gamma-glycidoxy-propyltrimethoxysilane, beta-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, tetramethoxysilane, tetraethoxysilane, tetrapropoxysilane, tetrabutoxysilane, the partial hydrolysis condensation products of any of the preceding compounds and ethyl polysilicate. These compounds may be used individually or by mixing with each other. Based on the reactivity with the hydroxyl groups of the epoxy resin, low molecular weight organoalkoxysilanes such as methyltrimethoxysilane and ethyltrimethoxysilane, are desirable. Also, based on their excellent effect in improving the adhesion with the base material, gamma-mercaptopropyltrimethoxysilane and gamma-glycidoxypropyltrimethoxysilane, are desirable. The epoxy resin (b), the other component of (A), must have at least one hydroxyl group and at least one epoxy group per molecule. Either the bisphenol type epoxy resins or the novolak type of epoxy resins are usable. The bisphenol resins are preferred. In particular, epoxy resins obtained by the condensation of bisphenol A and epichlorohydrin are preferred. These are expressed by the average formula ##STR1## A hydroxyl equivalence in the range of 100 to 220 is desirable. When the hydroxyl equivalence is less than this range, the quantity of component (b) modified by the silicone compound becomes small, resulting in difficulty in the formation of a satisfactory film. When the hydroxyl equivalence is greater than 220, under the ordinary conditions of the condensation reaction of components (a) and (b), unreacted hydroxyl groups tend to remain which may lower the storage stability when the organotitanium acid ester of component (B) is present in (A). Also, in the condensation reaction this will cause problems of increased viscosity and gelation. Furthermore, although the epoxy groups generally do not participate in the condensation reaction with the alkoxy groups of component (a) and hydroxyl groups of component (b), an epoxy equivalence in the range of 180 to 4,000 is desirable in order to increase the adhesive effect of the primer composition. It is preferred that average molecular weight be 300 to 3,000 and particularly 700 to 1,400. Component (A) can be obtained by mixing the above-mentioned components (a) and (b) at a temperature above the boiling point of the by-produced alcohol. The two components condense liberating the alkoxy groups of component (a) and the hydroxyl groups of component (b) as alcohol. Generally, it is easier to carry out this reaction using no catalyst or a small amount of a condensation catalyst at 80.degree. to 160.degree. C. with continuous removal of the liberated alcohol. A solvent or diluent such as toluene, xylene or ethyl acetate may be used during the reaction. It should be noted that after the reaction, no hydroxyl groups should remain on the epoxy resin. For this reason, it is advantageous to use a small amount of a catalyst. During the condensation of components (a) and (b), it is desirable that the reaction be carried out under the conditions wherein the ratio of mols of alkoxy groups in component (a) to the mols of hydroxyl groups in component (b) is 1 or greater. When the reaction is carried out at below 1 and especially when the organotitanium acid ester of component (B) is added to component (A), gelation is easily effected. Thus, in order to obtain a durable, homogeneous liquid primer composition, the reaction should be carried out at the above-mentioned molar ratios. It is preferred that component (A) contain at least 3 alkoxy groups.
Component (B), the organotitanium acid ester, after reacting with the alkoxy groups of component (A), will not only cure the primer composition and provide air dryability, but also will remarkably improve the adhesive strength, especially the adhesive durability, between the substrate and the room temperature-curable silicone rubbers or room temperature-curable silicone-modified organic rubbers. Examples of titanium compounds useful in this invention are as follows: tetraisopropyl titanate, tetra-n-butyl titanate, butyl titanate dimer, tetra(2-ethylhexyl) titanate, diethoxytitanium acetylacetonate, titanium diacetylacetonate, octylene glycol titanate, titanium lactate, titanium ethyl lactate, ethanolamine titanate, titanium chelates such as beta-diketonate chelates of dialkoxytitaniums, ketoacid ester chelates of dialkoxytitaniums or their partial hydrolysis condensation products. Component (B) may be used as individual compounds or mixtures of individual compounds. The amount of Component (B) useful in this invention is 0.1 to 100 parts by weight per 100 parts of component (A). From the viewpoint of curability, adhesion and storage stability, 5 to 25 parts by weight is desirable.
Simple mixing of these components will provide the primer composition of this invention.
Furthermore, in an attempt to improve the adhesion, especially the adhesion durability of the primer, and also to improve the air dryability and thus increase the productivity, the addition of an additional silane or its partial hydrolysis condensation product expressed by the following average formula, is effective: EQU R.sup.4 Si(OR.sup.5).sub.3
wherein R.sup.4 represents organic groups such as alkyl groups such as methyl and ethyl; monovalent unsaturated aliphatic hydrocarbon groups such as vinyl and methacryloxy and organofunctional groups such as glycidyl and mercaptopropyl; R.sup.5 is represented by alkyl groups such as methyl, ethyl and propyl or alkoxyalkyl groups. Several examples of these compounds are methyltrimethoxysilane, dimethyldiethoxy-silane, ethyltriethoxysilane, phenyltrimethoxysilane, vinyltrimethoxysilane, allyltripropoxysilane, gamma-methacryloxypropyltrimethoxysilane, glycidoxypropyltrimethoxysilane, beta-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, tetrapropoxysilane, tetrabutoxysilane and the partial hydrolysis and condensation products of the preceding compounds and ethyl polysilicate. The silane or partial hydrolysis condensation product should be selected according to the intended application of the primer composition.
When applying the primer composition to the substrate and the viscosity is too high or a thin film is desired, it is possible to use an organic solvent for dilution. For example, toluene, xylene or ethyl acetate may be used as the organic solvent. In an attempt to further increase the postcure film strength, various inorganic fillers may be added to the primer compositions, for example, a powdered silica may be added. Other fillers suitable for this purpose are hydrophobic silicas with a trimethylsilylated surface. Moreover, any generally well known heat stabilizer or colorants such as red iron oxide, cerium oxide, fatty acid salts of iron, titanium oxide or other agents may be optionally added provided interference with the purpose of this invention is not caused.
The primer of this invention is suitable for use as a pre-process treatment for substrates in order to increase the adhesion and durability of room temperature-curable silicone rubbers and room temperature-curable silicone-modified organic rubbers with various substrates throughout the curing process. The room temperature-curable silicone rubber may be a single package or two package type. Any type involving liberation of alcohol, oxime, ketone, amine, hydroxylamine or carboxylic acid may be used. The room temperature-curable silicone-modified organic rubbers may also be single or two package types. Examples are as follows: alkoxysilyl-terminated polyether rubbers, alkoxysilyl-terminated polybutadiene rubbers and alkoxysilyl-terminated polyurethane rubbers.
By using these primer compositions as a pre-process treatment for poorly adhering base materials such as pure aluminum, surface-treated aluminum, coated aluminum, stainless steel, mortar and concrete, the above-mentioned rubber can be adhered firmly and durably. The sealing of different substrates in construction can be carried out smoothly.
Materials similar to those discussed above are also appropriate for use as coatings for various substrates. Therefore, this invention also deals with coating compositions. More particularly, this aspect of the invention deals with compositions which can be stored over a long period of time in the absence of moisture but can be cured at room temperature in the presence of moisture as coatings. These compositions exhibit excellent adhesion to various substrates.
Generally, silicones exhibit excellent heat resistance and electrical insulating properties and they have therefore been widely used for heat-resistant paints and varnishes for electrical insulation. However, silicones have some disadvantages. For example, silicones require high temperatures and long term heating for curing. In addition, they can be adhered satisfactorily to metal plates such as soft steel and stainless steel, but they cannot be easily adhered to plastics.
The inventors pursued the present investigation in an attempt to overcome the above-mentioned disadvantages of silicones. It was found that compositions can be obtained which can be stored for long periods of time in the absence of moisture but which can be cured at room temperature in the presence of moisture to form coatings which exhibit excellent adhesiveness with a variety of substrates such as metals and plastics.
This invention therefore deals with coating compositions which consist of a coating composition comprising (I) 100 parts by weight of a silicone modified epoxy resin containing silicon-bonded alkoxy groups wherein the modified epoxy resin is obtained by contacting and reacting a compound (a) containing at least two silicon-bonded alkoxy groups per molecule having the unit formula ##STR2## wherein R.sup.6 is a substituted or unsubstituted monovalent hydrocarbon radical, X' is an alkoxy radical having the formula R.sup.7 O-- wherein R.sup.7 is an alkyl radical of 1 to 4 carbon atoms or the radical R.sup.8 OR.sup.9 -- wherein R.sup.8 is an alkyl radical of 1 to 4 carbon atoms and R.sup.9 is divalent alkylene radical of 1 to 3 carbon atoms; c has a value of 0 to 2; d has a value of 1 to 4 and the sum of c+d has a value of 1 to 4, with (b) an epoxy resin containing at least one epoxy group and at least one hydroxyl group per molecule and wherein prior to contacting and reacting components (a) and (b), the ratio of alkoxy groups in component (a) to the hydroxy groups in component (b) is equal to or greater than 1; and (II) 0.1 to 100 parts by weight of an organotitanium compound.
Component I of this invention is prepared from two reactive materials I(a) and I(b). Specifically, Component (I) of the composition is a product of the condensation reaction between the silicon-bonded alkoxy groups in component I(a) and the hydroxyl groups in component I(b). This material is a primary component of the coating compositions of this invention. Component I(a) is a silane or polysiloxane containing at least two silicon-bonded alkoxy groups per molecule and has the unit formula ##STR3## The alkoxy groups undergo a condensation reaction with the hydroxyl groups in the epoxy, component I(b). In the formula, R.sup.10 represents substituted or unsubstituted monovalent hydrocarbon radicals bonded to silicon atoms. Examples of these groups are alkyl groups such as methyl, ethyl, propyl and octadecyl; alkenyl groups such as vinyl and allyl; aryl groups such as phenyl and naphthyl and their derivatives in which some of the hydrogen atoms are substituted with halogen atoms, cyano groups, hydroxyl groups or mercapto groups or in which some of the hydrogen atoms on alkyl groups are substituted with functional groups such as methacryloxy acryloxy, glycidyl and 3,4-epoxycyclohexyl. X" for purposes of this invention is a silicon-bonded alkoxy group represented by the formula R.sup.11 O-- wherein R.sup.11 is an alkyl radical of 1 to 4 carbon atoms or the radical R.sup.12 OR.sup.13 -- wherein R.sup.12 is an alkyl radical of 1 to 4 carbon atoms and R.sup.13 is a divalent alkylene radical of 1 to 3 carbon atoms. Examples of these groups are methoxy, ethoxy, propoxy and methoxyethoxy. For purposes of this invention, e has a value of 0 to 2 and f has a value of 1 to 4 and the sum of e+f has a value of 1 to 4. The fact that e has a value of 0 to 2 indicates that R.sup.10 is not necessarily present in component I(a). However, e must be 2 or less and f must be 1 or more. Each molecule must contain at least two silicon-bonded alkoxy groups. The reason for this is that if the number of alkoxy groups is too low, the degree of condensation with the hydroxyl groups in component I(b) will be decreased and the number of silicon-bonded alkoxy groups in component (I) will be less which results in insufficient curing. In this sense, it is desirable that at least three X' groups be present in component (I) after the reaction of components I (a) and I (b).
Component I(a) can be either a silane or a polysiloxane. If it is a polysiloxane, the degree of polymerization must be 2 or greater. The molecular configuration of the polysiloxane can be linear, branched chain or network. In addition to the reactive alkoxy groups, it can contain small amounts of silicon-bonded hydroxyl groups, halogen atoms or hydrogen atoms. Examples of component I(a) are silanes such as methyltrimethoxysilane, methyltripropoxysilane, dimethyldiethoxysilane, ethyltri-ethoxysilane, phenyltrimethoxysilane, diphenyldimethoxysilane, phenylmethyldiethoxysilane, vinyltrimethoxysilane, allyltripropoxysilane, vinyltri(methoxyethoxy)silane, methylvinyldiethoxysilane, tetramethoxysilane, tetraethoxysilane, gamma-chloropropyltrimethoxysilane, gamma-chloropropylmethyldimethoxysilane, gamma-glycidylpropyltrimethoxysilane, gamma-glycidylpropylmethyldiethoxysilane, beta-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, gamma-methacryloxypropyltrimethoxysilane, gamma-methacryloxypropylmethyldiethoxysilane and gamma-acryloxypropyltrimethoxysilane; partial hydrolysis and condensation products of one or two or more of these silanes; partial hydrolysis and condensation products of these silanes with other organosilanes such as trimethoxyethoxysilane and ethyl polysilicate.
Component I(b) is the same as that used for the primer composition discussed above and is an epoxy resin containing at least one epoxy group and hydroxyl group per molecule. Both bisphenol type resins and novolak type resin are applicable. Component I(b) preferably possesses the following structural formula with a molecular weight ranging from 300 to 6000. ##STR4## Examples of commercial epoxy resins useful herein are Epikote.RTM. 815, 820, 828, 834, 864, 1001, 1004 and 1007, all products of the Shell Chemical Corporation.
Component (I) is a silicone-modified epoxy resin containing silicon-bonded alkoxy groups which is obtained by a condensation reaction between components I(a) and I(b) wherein the ratio of alkoxy groups in component I(a) to the hydroxyl groups in component I(b) is equal to or greater than 1. If this ratio is less than 1, gelation is likely to occur during the condensation reaction between components I(a) and I(b). Although gelation may not occur during the condensation reaction, gelation is likely to occur when the organotitanium compound, component (II), is added thereto and a satisfactory storage stability therefore cannot be achieved. As the above-mentioned ratio increases, the chances of gelation occurring during the reaction becomes less. In addition, the storage stability of the coating composition, when component (II) is added, increases.
The condensation reaction between components I(a) and I(b) is preferably carried out at 80.degree.-250.degree. C. using a small amount of a conventional catalyst such as organotitanium acid esters. When this condensation reaction is carried out, an organic solvent which can dissolve both components I(a) and I(b) can be used. However, the use of an organic solvent is undesirable for some coating applications owing to deterioration of the substrates by the solvent. In such cases the reaction is preferably carried out in the presence of an excess of low boiling alkoxysilane.
Component (II) is a compound which makes component (I) room-temperature curable in the presence of moisture. Examples of such compounds: titanium acid esters of monohydric alcohols such as methanol, ethanol, isopropanol, butanol, cyclohexanol, octanol and octadecyl alcohol; titanium acid esters of dihydric alcohols such as ethylene glycol, propylene glycol, octylene glycol, diethylene glycol, tripropylene glycol and tetraethylene glycol; titanium acid esters of trihydric alcohols such as glycerin; alkoxy titanium chelates such as diisopropoxybis(acetylacetonato)titanium and di-n-butoxybis(triethanolaminato)titanium and dihydroxybis(lactato)titanium.
In terms of the amount of component (II) to be added, the curing rate slows down when the amount of component II is too small. In contrast, if too much is added, the cured film becomes brittle and cracks are easily produced. Thus, an appropriate amount ranges from 0.1 parts by weight to 100 parts by weight per 100 parts by weight of component (I). From a consideration of curability, adhesiveness and storage stability, the amount preferably ranges from 5 to 90 parts by weight.
The coating compositions of this invention can be obtained from components (I) and (II) by simply mixing them together. However, in order to increase the storage stability, both components are preferably mixed together in the absence of moisture and the product is stored in a moisture-impermeable container after mixing and degassing treatments.
Optionally, additional components such as organic solvents, inorganic fillers, pigments and heat-resisting agents can be added. In particular, additional organotrialkoxysilane such as has been disclosed for component I(a) can be added in an amount of 5 to 500 parts by weight per 100 parts by weight of component (II) after components I(a) and I(b) have been reacted and after component II has been added. The organic radicals in the organotrialkoxysilane are identical to the above-mentioned R.sup.10 radicals and the alkoxy radicals are also identical to the above-mentioned X". The coating compositions of this invention can be stored over a long period in the absence of moisture. In the presence of moisture, curing occurs even at room temperature to form a strong heat-resistant and weather-resistant film which is firmly adhered on substrates such as various types of metals, plastics and woods. Thus, the compositions are very useful as protective coating agents for these substrates.
Several examples illustrating the invention follow: The viscosity unless indicated otherwise was measured at 25.degree. C.
"Parts" in the examples denotes "parts by weight." The finger touch drying time, adhesiveness and pencil hardness were determined under the following conditions:
Finger touch drying time: the finger touch drying time was defined by the time in which the coated surface ceases to be tacky after being coated on the substrate and a fingerprint no longer imprints on the surface. Pencil hardness: the pencil hardness was measured according to JIS K 5400.
Adhesiveness (Cross-cut adhesion test): one hundred, 1 mm squares, were cut in a 1 cm square of the cured coating using a razor blade and a commercial cellophane tape was firmly pressed and adhered over the cut squares. The number of squares remaining out of 100 squares was counted after the tape had been quickly peeled. The number of remaining squares was recorded as the numerator and the number of cut squares was recorded as the denominator.